1. Field of the Invention
The present invention relates to piezoelectric resonators and electronic components containing the same, and more particularly, to a piezoelectric resonator which maximizes the effective use of the mechanical resonance of a piezoelectric member, and electronic components using the piezoelectric resonator, such as an oscillator, a discriminator, and a filter.
2. Description of the Related Art
FIG. 39 is a perspective view of a conventional piezoelectric resonator. A piezoelectric resonator 1 includes a single piezoelectric substrate 2 having, for example, a rectangular plate shape as viewed from above. The single piezoelectric substrate 2 is polarized in the thickness direction. On two opposite major surfaces of the single piezoelectric substrate 2, electrodes 3 are provided. When a signal is input between the electrodes 3, an electrical field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in the longitudinal direction. In FIG. 40, there is shown a piezoelectric resonator 1 in which electrodes 3 are provided on two opposite major surfaces of a single piezoelectric substrate 2 having a square plate shape as viewed from above. The single piezoelectric substrate 2 of the piezoelectric resonator 1 is polarized in the thickness direction. When a signal is input between the electrodes 3 in the piezoelectric resonator 1, an electrical field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in a square-type vibration mode (in the plane direction).
These piezoelectric resonators shown in FIGS. 39 and 40 are of an unstiffened type, in which the vibration direction differs from the direction of polarization and the electrical field direction. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator in which each of the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same.
An unstiffened piezoelectric resonator has a relatively small frequency difference .DELTA.F between the resonant frequency and the anti-resonant frequency. This leads to a drawback in which a frequency-band width in use is narrow when an unstiffened frequency resonator is adapted to be used as an oscillator or a filter. Therefore, the degree of freedom and flexibility in resonator characteristics design is low in such a piezoelectric resonator and electronic components including such resonators.
The piezoelectric resonator shown in FIG. 39 uses the first-order resonance in the longitudinal mode. Because of its structure, the piezoelectric resonator of FIG. 39 also generates large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and in a width mode. To suppress these spurious resonances, some solutions have been considered, such as polishing, increasing mass, and changing the shape of the electrode. These solutions increase manufacturing cost.
In addition, since the single piezoelectric substrate has a rectangular plate shape, the single substrate cannot be made thinner without sacrificing required strength. Therefore, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be increased. This makes it extremely difficult to achieve impedance matching with an external circuit. To form a ladder filter by alternately connecting a plurality of piezoelectric resonators in series and in parallel, the capacitance ratio of the series resonator to the parallel resonator needs to be made large in order to increase attenuation. Because a piezoelectric resonator has the shape and structural restrictions described above, however, large attenuation cannot be obtained.
In the piezoelectric resonator shown in FIG. 40, large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are generated. Since the piezoelectric resonator must have a large size as compared with a piezoelectric resonator using the longitudinal vibration in order to obtain the same resonant frequency, it is difficult to reduce the size of the piezoelectric resonator shown in FIG. 40. When a ladder filter is formed by a plurality of piezoelectric resonators, in order to increase the capacitance ratio between the series resonator and the parallel resonator, the resonators connected in series must have an increased thickness and electrodes are formed only on part of a piezoelectric substrate to make the capacitance small. In this case, since the electrodes are only partially formed, the difference .DELTA.F between the resonant frequency and the anti-resonant frequency as well as the capacitance is reduced. The resonators connected in parallel are accordingly required to have small .DELTA.F. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission band width of the filter cannot be increased.
In the prior art, such as U.S. Pat. No. 5,250,868, a piezoelectric effect device includes a sintered stack of piezoelectric elements which stack is completely enclosed in a flexible metal housing to be used as an actuator. The stack includes non-active portions I.sub.1 and I.sub.n located at the ends of the stack for protecting the active portions I.sub.2 through I.sub.n-1. In order to function as an actuator, the stack 3 of piezoelectric elements must be supported at one end and must be free to vibrate in a longitudinal mode at the other end. Thus, the stack 3 could not be supported at a point other than the one end and therefore, the stack 3 could not be supported at a central or middle portion of the stack 3 without destroying the operability of the stack 3 as an actuator. In addition, the stack 3 of piezoelectric elements must achieve maximum displacement required for actuators and therefore, the stack 3 must have a node point located only at the supported end of the stack 3. The stack 3 cannot have a node point located at a central or middle portion of the stack, otherwise the stack 3 could not function as an actuator. By arranging the stack 3 to have a node point at one end and to be supported at the same end, the stack 3 can achieve the maximum desired displacement necessary in an actuator.
In addition, U.S. Pat. No. 4,633,120 teaches an electrostriction transducer including a plurality of electrostriction layers, not piezoelectric layers, stacked on each other and including protection or dummy layers on each end of the stack of electrostriction layers for protecting the stack of electrostrictive layers. The electrostriction layers are very different from piezoelectric elements and cannot function in the same manner as piezoelectric elements, as is well known.
The prior art stacked devices are only adapted to function as an actuator or electrostrictive transducer, and could not function as a resonator. Therefore, a location of a resonant point and an anti-resonant point and an amount of capacitance is of no concern in these types of devices. Accordingly, the prior art stacked devices have electrodes provided at both ends of the stack of piezoelectric or electrostriction members. These electrodes cannot be located along a longitudinal side edge of the stack in order for the stacked devices to function as an actuator or transducer. Furthermore, these prior art devices cannot have a node located at a central or middle portion of a stack of piezoelectric elements or be supported at a node located at a central or middle portion of the stack.
In addition, the only purpose for providing the protection or dummy layer is to protect the piezoelectric or electrostriction layers surrounded by the protection or dummy layers. The stacked piezoelectric or electrostrictive layers of the prior art are not arranged or adapted to function as a resonator which can be used in an oscillator, a discriminator, or a filter.